A fuel cell is an electrochemical cell comprising two catalysed electrodes separated by an electrolyte. A fuel, especially hydrogen (including hydrogen-containing “reformate”) or methanol, is supplied to an anode, and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted into electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine (including gas turbines) in stationary and automotive applications or energy storage batteries in portable power consuming devices. The first bulk applications of fuel cell stacks are now on the market as auxiliary power sources for high-end boats and recreational vehicles. Extensive research into fuel cells continues, and fuel cells are being mooted as battery replacements to provide increased energy density power sources in laptop-type computers, mobile phones and similar small electronic devices.
The principal type of fuel cell is the Polymer Electrolyte Membrane (PEM) cell. In this, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton-conducting membranes based on perfluorosulphonic materials are typically used, although many other membranes are being investigated. Protons produced at the anode are transported across the membrane to the cathode, where they combine with oxygen to produce water.
The main component of the PEM fuel cell is the membrane electrode assembly (MEA) and a state of the art MEA has five layers. The central layer is a polymer membrane, and on either side of the membrane is an electrocatalyst layer which is tailored for the different requirements at the anode and the cathode. Finally, adjacent each catalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and also must conduct the electric current that is generated by the electrochemical reactions. Therefore, the substrate must be porous and electrically conducting. The components are bonded and sealed together to form an MEA which is built up into complete cells together with rigid flow field plates which distribute fuel and oxidant gases and remove water. A number of cells comprising MEAs and their associated flow field plates are assembled together to form a fuel cell stack.
The MEA can be assembled by several methods known in the art. The electrocatalyst (“catalyst”) layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two such electrodes can be placed on either side of a membrane and laminated together. Another method is to coat the two catalysts on either side of the membrane to form a catalyst-coated membrane (CCM), apply a gas diffusion substrate to both faces of the catalyst-coated membrane, followed by laminating. A further method is a combination method, using a one-sided catalyst coated membrane with a gas diffusion substrate, and on the other side of the membrane, a gas diffusion electrode.
Despite the advances made in MEAs and fuel cells generally, there remains a need for alternative constructions offering yet further efficiencies, but also satisfying the requirement to further reduce costs and/or size of the fuel cell stack.